Hydrocarbons, principally oil and natural gas, are produced by drilling a well or borehole from the earth surface to a subterranean formation or zone which contains the hydrocarbons, and then flowing the hydrocarbons up the well to the earth surface. Natural formation pressure forces the hydrocarbons from the surrounding hydrocarbons-bearing zone into the well bore. Since water is usually present in most subterranean formations, water is also typically pushed into the well bore along with the hydrocarbons.
In the early stages of a producing well, there may be sufficient natural formation pressure to force the liquid and gas entirely to the earth's surface without assistance. In later stages of a well's life, the diminished natural formation pressure may be effective only to move liquid partially up the well bore. At that point, it becomes necessary to install pumping equipment in the well to lift the liquid from the well. Removing the liquid from the well reduces a counterbalancing hydrostatic effect created by the accumulated column of liquid, thereby allowing the natural formation pressure to continue to push additional amounts of liquid and gas into the well. Even in wells with low natural formation pressure, oil may drain into the well. In these cases, it becomes necessary to pump the liquid from the well in order to maintain productivity.
There are a variety of different pumps available for use in wells. Each different type of pump has its own relative advantages and disadvantages. In general, however, common disadvantages of all the pumps include a susceptibility to wear and failure as a result of frictional movement, particularly because small particles of sand and other earth materials within the liquid create an abrasive environment that causes the parts to wear and ultimately fail. Moreover, the physical characteristics of the well itself may present certain irregularities which must be accommodated by the pump. For example, the well bore may not be vertical or straight, the pipes or tubes within the well may be of different sizes at different depth locations, and the pipes and tubes may have been deformed from their original geometric shapes as a result of installation and use within the well. A more specific discussion of the different aspects of various pumps illustrates some of these issues.
One type of pump used in hydrocarbons-producing wells is a rod pump. A rod pump uses a series of long connected metal rods that extend from a powered pumping unit at the earth surface down to a piston located at the bottom of a production tube within the well. The rod is driven in upward and downward strokes to move the piston and force liquid up the production tube. The moving parts of the piston wear out, particularly under the influence of sand grain particles carried by the liquids into the well. Rod pumps are usually effective only in relatively shallow or moderate-depth wells which are vertical or are only slightly deviated or curved. The moving rod may rub against the production tubing in deep, significantly deviated or non-vertical wells. The frictional wear on the parts diminish their usable lifetime and may increase the pumping costs due to frequent repairs.
Another type of pump uses a plunger located in a production tubing to lift the liquid in the production tubing. Gas pressure is introduced below the plunger to cause it to move up the production tube and push liquid in front of it up the production tube to the earth surface. Thereafter, the plunger falls back through the production tube to the well bottom to repeat the process. While plunger lift pumps do not require long mechanical rods, they do require the extra flow control equipment necessary to control the movement of the plunger and regulate the gas and liquid delivered to the earth surface. The plunger must also have an exterior dimension which provides a clearance with the production tubing to reduce friction and to permit the plunger to move past slight non-cylindrical irregularities in the production tubing without being trapped. This clearance dimension reduces the sealing effect necessary to hold the liquid in front of the plunger as it moves up the production tubing. The clearance dimension causes some of the liquid to fall past the plunger back to the bottom of the well, and causes some of the gas pressure which forces the plunger upward to escape around the plunger. Diminished pumping efficiency occurs as a result. Plunger lift pumps also require the production tubing to have a substantial uniform size from the top to the bottom.
A gas pressure lift is another example of a well pump. In general, a gas pressure lift injects pressurized gas into the bottom of the well to force the liquid up a production tubing. The injected gas may froth the liquid by mixing the heavier density liquid with the lighter density gas to reduce the overall density of the lifted material thereby allowing it to be lifted more readily. Alternatively, “slugs” or shortened column lengths of liquid separated by bubble-like spaces of pressurized gas are created to reduce the density of the liquid, and the slugs are lifted to the earth surface. Although gas pressure lifts avoid the problems of friction and wear resulting from using movable mechanical components, gas pressure lifts frequently require the use of many items of auxiliary equipment to control the application of the pressures within the well and also require significant equipment to create the large volumes of gas at the pressures required to lift the liquid.
At some point in the production lifetime of a well, the costs of operating and maintaining the pump are counterbalanced by the diminished amount of hydrocarbons produced by the continually-diminishing formation pressure. For deeper wells, more cost is required to lift the liquid a greater distance to the earth surface. For those wells which require frequent repair because of failed mechanical parts, the point of uneconomic operation is reached while producible amounts of hydrocarbons may still remain in the well. For those deep and other wells which require significant energy expenditures to pump, the point of uneconomic operation may occur earlier in the life of a well. In each case, the hydrocarbons production from a well can be extended if the pump is capable of operating by using less energy under circumstances of reduced requirements for maintenance and repair.